Structures such as grooves and the like are formed on a silicon substrate by a dry etching process. For example, in the field of semiconductor integrated circuits, there is constant progress towards higher integration and higher density products. An etching technology which can form trenches (deep grooves or deep holes) with high precision is sought. An example of an etching method for this type of trench etching is disclosed in Japanese Laid Open Patent Publication 7-503815.
With this etching method, an etching mask of the desired shape is formed on the silicon substrate surface. Next, there is an etching step for forming grooves or holes in which a mixture gas of SF6 and Ar changed into plasma is used for dry etching of the substrate surface. Similarly, using a mixture gas of CHF3 and Ar changed into plasma, there is a polymerization step (proactive film formation step) which forms a protective film on the side walls of the grooves or holes (henceforth referred to as grooves or the like). By alternately repeating these steps, deep grooves or deep holes (henceforth referred to as deep grooves or the like) are formed.
With this etching method, the wall surfaces of the grooves and the like formed in sequence through dry etching is covered afterwards with a protective film. During subsequent dry etchings, the previous wall surfaces are protected by this protective film. As a result, any extreme side etching or undercutting is prevented, and grooves and the like with wall surfaces which appear to be vertical are formed.
However, with the above etching method, there were the following problems. With the above etching method, there is an etching step which does not include the formation of a protective film on the wall surface, and there is a step for formation of a protective film on the wall surface. These two steps are alternately repeated. As a result, the new wall surfaces formed by etching do not have a protective film. Because of this, in the etching step, the wall surfaces are etched together with the etching ground surface (the bottom surface of the groove or the like). As a result, the wall surfaces have an undulating surface in the vertical direction, and the processing precision is poor. This unevenness of the wall surfaces becomes an obstacle for achieving high integration and high density in the field of semiconductor integrated circuits.
The present inventors have proposed an etching method in Japanese Patent Application Number 2001-299435 in which both the etching step and protective film formation step are implemented using a mixture gas of etching gas and protective film formation gas while having a bias voltage by constantly applying power to the silicon substrate. During the etching step, a mixture gas of a large amount of etching gas and a small amount of protective film formation gas is used. In the protective film formation step, a mixture gas having a small amount of etching gas and a large amount of protective film formation gas is used.
According to this etching method, the etching step and protective film formation step are implemented using a mixture gas of etching gas and protective film formation gas. As a result, in the etching step, the etching ground is etched by the etching gas, and in addition, the vertical structural surfaces formed by etching are immediately coated with a protective film by the protective film formation gas. In the protective film formation step occurring next, the vertical structural surfaces are further firmly covered by a protective film. With this, the etching of the vertical structural surface is suppressed, and the unevenness in its surface is small. Vertical structural surfaces with excellent perpendicularity are formed.
In addition, because power is constantly being applied to provide a bias voltage to the silicon substrate, the etching ground can be physically etched by ion irradiation. In the etching step, the etching speed is increased, and in the protective film formation layer step, the formation of a protective film in the etching ground surface is prevented. As a result, this method has the advantage of being able to shorten the overall etching time.
However with this etching method, as described above, a mixture gas of an etching gas and a protective film formation gas is used for etching. As a result, although the vertical structural surfaces formed by etching are effectively protected, at the etching ground surface, there is the simultaneous progression of opposing actions of etching by the etching gas and ion irradiation and also the formation of a protective film which suppresses etching. Therefore, energy for etching is also spent on removing the protective film. Compared to etching methods which do not use protective film formation gas during etching, the etching speed is reduced with this method.
In addition, with regard to the protective film formation step, the formation of a protective film on the etching ground surface can be suppressed by the etching gas and ions, and the overall etching time can be shortened. However, despite these advantages, because the etching gas also acts on the vertical structural surfaces, these vertical structural surfaces can become etched. In some cases, these surfaces cannot be made adequately smooth.
The present inventors conducted extensive research on the mixture proportions of the mixture gas. This resulted in the discovery of an optimal mixing ratio which increases etching speed and results in the etching formation of vertical structural surfaces with adequate smoothness and excellent perpendicularity as compared to etching methods in which protective film formation gas is not used in the etching progression step and etching gas is not used in the protective film formation step.